The present invention relates to cold cathode fluorescent lamps, and more specifically, to a method and apparatus for extending the life of cold cathode fluorescent lamps operating at full front surface luminance when used for backlighting flat panel displays.
Either hot cathode or cold cathode fluorescent lamp systems are used to illuminate liquid crystal flat panel displays. The choice between hot or cold cathode fluorescent lamps is selected based on the brightness, efficiency, and dimming desired. Hot cathode fluorescent lamps' typical life span is approximately 20,000 hours, with end-of-life being sudden. The sudden end-of-life is caused by the unavoidable loss of barium in the emission mix material that covers the electrodes. To assure thermionic emission of electrons in dimming applications, additional power is generally needed to maintain optimum electrode hot-spot temperature. Even in applications with optimized filament heating, the predominant lamp failure mechanism is filament failure. The hot cathode life cannot be significantly extended even if operation is at a much lower current. Cold cathode fluorescent lamps do not have these deficiencies.
Unlike hot cathode lamps, cold cathode lamps have large metal electrodes that do not require additional heating. Both hot and cold cathode lamps experience a slow degradation of phosphor conversion efficiency. Unique to cold cathode lamps is a gradual darkening of the lamps' ends which eventually spreads to the whole tube. This darkening is due to ion bombardment of the electrodes and subsequent sputtering of electrode metal material to the wall. The life of cold cathode lamps is measured as a half-brightness life when operating at full-on constant power. The typical half-life for cold cathode lamps is 20,000 hours.
A known solution to lamp luminance degradation is allowing the user dimming capability. Dimming is typically accomplished through either current limiting or pulse width modulation (PWM). In the current limiting mode, the lamp current is reduced, but the lamp stays on all the time. In the PWM mode, the lamp is turned full on and off at a repetition rate of about 100 to 400 Hz with a dimming range being determined by the duty cycle (fraction on time). A problem with the prior art is that the user controls the brightness, enabling full-on initial brightness. Full brightness may be more than required in the system design. Operating at full power will result in an accelerated luminance depreciation due to phosphor degradation. If the design utilizes hot cathode lamps, no extension of lamp life could be expected if lamp dimming is used.
As an illustration, consider a backlight application in a flat panel display where the display surface luminance requirement is 100 footlamberts (fl). If the designer chooses either a hot cathode or cold cathode lamp with a resulting maximum front surface luminance value of 100 fl, the luminance will gradually drop over time, and the front surface luminance will have less than the desired 100 fl. In this application, the cold cathode loss rate will typically be higher than the hot cathode loss rate, but the hot cathode failure would be sudden. The cold cathode lamps would simply get dimmer. If the designer chooses higher rated brightness lamps but does not scale the current down, the initial full-on brightness would be greater than desired, but would decay over time eventually reaching the desired 100 fl and then decay beyond this point due to phosphor degradation during the normal life of the lamp. In other words, no extended life of the lamp is realized. Accordingly, it would be advantageous to provide a method and system to significantly extend the life of cold cathode lamps.